Minimizing N-Nitrosodimethylamine Formation During Disinfection of Blended Seawater and Wastewater Effluent

Augmenting seawater with wastewater has the potential to reduce the energy demand and environmental impacts associated with seawater desalination. Alternatively, as wastewater reuse becomes more widespread, augmenting wastewater with seawater can increase the available water supply. However, the chemistry of disinfecting a blended stream has not been explored. Toxic byproducts, including N-nitrosodimethylamine (NDMA), are expected to form during disinfection, and the extent of formation will likely be a function of which stream is chlorinated and whether disinfection happens before or after blending. In this work, three blending-disinfection scenarios were modeled and experimentally evaluated in bench-scale systems treating synthetic and authentic waters. Modeling results suggested that chlorinating preblended wastewater and seawater would produce the most NDMA because it yielded the highest concentrations of bromochloramine, which was previously found to promote NDMA formation. However, chlorinating wastewater prior to blending with seawater, which modeling indicated would form the most dichloramine, produced the most NDMA in experiments. When seawater was disinfected prior to blending with wastewater, bromide likely converted most chlorine to free bromine. Bromamines formed after blending, however, did not lead to an elevated level of NDMA formation. Therefore, to minimize NDMA formation when disinfecting blended wastewater–seawater, seawater should be disinfected prior to introducing wastewater.


Contents
Text S1: NDMA extraction, quantification, and detection limit determination…………………S3 Table S1: Chemical equations and rate constants applied to Kintecus model………….….S4-S5 Table S2: Typical bromide, chlorine, ammonia, and chloride concentrations of secondary treated wastewater and seawater…………………………………………………………………………S6 Table S3: Typical ionic strength values of secondary treated wastewater and seawater………….S7 Table S4: Typical pH values of secondary treated wastewater and seawater……………………..S7 Table S5: Composition of secondary treated wastewater and seawater used in bench-scale disinfection experiments…………………………………………………………………………S8 Table S6: QA/QC data to determine detection limit of NDMA using EPA Method 552.3………S8Text S1.NDMA extraction, quantification, and detection limit determination Extraction and quantification.NDMA analytical standards were prepared by spiking a range of NDMA concentrations from a 5 mg/L stock solution (prepared in dichloromethane) into 23.5 mL of 10 mM pH 7 phosphate buffer prepared in Milli-Q water in a 23.5 mL borosilicate vials capped with PTFE-faced septa.Standards were spiked with 10 µg/L of d6-NDMA (as a masslabeled surrogate) from a 2 mg/L stock (prepared in acetonitrile).Standards were extracted in the same manner as experimental samples: 23.5 mL samples were transferred to 40 mL borosilicate vials containing 10% v/v (2.35 mL) of dichloromethane, capped with PTFE-faced septa, and shaken for two minutes.After phase separation, the dichloromethane phase was removed, dried with magnesium sulfate heptahydrate, and transferred to 2 mL autosampler vials for GC/MS/MS analysis.An example extracted calibration curve is provided in Figure S6A.Extraction efficiency.Extraction efficiency was calculated as the volume-normalized average peak area ratio of d6-NDMA in six extracted standards compared to six standards prepared at the same concentration in dichloromethane (Figure S6B): 5  6 = 35% Limit of detection.The method detection limit was determined by preparing seven replicate 23.5 mL standards in 10 mM phosphate buffer (pH 7) at a concentration estimated to produce a signal-to-noise ratio of 5:1, and extracting and analyzing in the same manner as experimental samples.The standard deviation of the seven concentrations was multiplied by the Student's ttest value corresponding to a 99% confidence interval (3.143) to produce the detection limit (2.8 ng/L) (Table S6) [1].

Figure S5 .
Figure S5.Cumulative halamine exposures modeled as a function of wastewater fraction for (a) seawater disinfection prior to blending, (b) wastewater disinfection prior to blending, and (c) blended disinfection.

Table S2 :
Typical bromide, chlorine, ammonia, and chloride concentrations of secondary treated wastewater and seawater.Concentrations used in "This study" refers to the value used for modeling experiments and synthetic experiments.

Table S3 :
Typical ionic strength values of secondary treated wastewater and seawater."This study" refers to the value used for synthetic water experiments.

Table S4 :
Typical pH values of secondary treated wastewater and seawater."This study" refers to the value used for synthetic water experiments.

Table S5 .
Composition of secondary treated wastewater and seawater used in bench-scale disinfection experiments.

Table S6 .
NDMA limit of detection determination.